1. Field of Invention
This invention relates to systems for directing or controlling energy. Specifically, the present invention relates to isolators for selectively blocking, redirecting, or absorbing reflected energy, such as microwave energy.
2. Description of the Related Art
Isolators are employed in various demanding applications including communications, space-based remote sensing systems, and military avionics. Such applications demand efficient and cost-effective isolators that can accommodate large amounts of reflected power without damage.
In relatively low-frequency applications involving radio-frequency waves, wave reflections are often attenuated via ferrite-based microstrip or waveguide circulators and isolators. Microstrip implementations of these devices have relatively low power-handling capability, while waveguide-based devices are often unacceptably lossy at multi-kilowatt power levels and frequencies beyond 10 GHz. Ultimately, the power-handling capabilities of waveguide-based isolators and circulators are limited by the dielectric-breakdown limit or air-breakdown limit, which is the electric field strength at which the dielectric or air in the waveguide is ionized. For example, at 95 GHz, a WR-8 waveguide having a cross-section measuring 80 mils (1 mil=0.001 inch) in width and 40 mils in height has a theoretical maximum continuous wave power rating of less than 2.6 kW.
Alternatively, low-power quasioptical isolators having capacitively-loaded linear-to-circular polarization converter grids, capacitively-loaded dipole tuner grids, and resistively-loaded absorber grids are employed. Resistive and capacitively-loaded dipole tuner grids are employed to absorb reflected energy having a predetermined polarization as described by Hollung et al. in “A Quasi-Optical Isolator,” IEEE Microwave and Guided Wave Letters, Volume 6, pages 205–206, published in May 1996. Unfortunately, these capacitively and resistively-loaded grids have limited power-handling capabilities.
Alternatively, circular polarization duplexers, such as those described by Nakajima and Watanabe in “A Quasioptical Circuit Technology for Short Millimeter-Wavelength Multiplexers,” IEEE Trans. Microwave Theory and Techniques MTT-29, pages 897–905, published in September 1981, are employed as isolators in low-power applications. Such duplexers employ a wire grid beamsplitter followed by a quarter-wave plate constructed from a dielectric. The quarter-wave plate and beamsplitter are configured so that reflected energy passing back through the quarter-wave plate exhibits a polarization that is reflected by the beamsplitter. Such duplexers, however, are limited to relatively low-power applications, since the dielectric quarter-wave plate has insufficient heat dissipation capabilities for many high-power applications. Furthermore, a high-power incident beam whose electric field is parallel to the wires in the wire-grid beamsplitter will induce large currents in the narrow wires of the beamsplitter, which may cause the beamsplitter to overheat and fail.
Isolators are particularly important in high-power Continuous Wave (CW) microwave/millimeter wave applications, which currently lack mechanisms to block reflected energy, and where reflected energy can damage or destroy microwave sources. Existing systems employing high-power millimeter-wave sources cannot protect expensive vital components from high-amplitude reflections. Unprotected millimeter-wave sources, such as gyrotron oscillators, may experience output window breakage if they are not sufficiently protected from reflected energy. Unfortunately, suitable quasioptical millimeter-wave isolators capable of handling hundreds of kilowatts of average power are typically unavailable.
High-power millimeter-wave sources, such as gyrotron oscillators, may have continuous-wave output power exceeding 100 kW. Such systems demand robust isolation to prevent reflected energy from damaging expensive and sensitive source components.
Hence, a need exists in the art for an efficient system and method that can effectively protect components from high-amplitude energy reflections, such as high-power millimeter wave reflections. There exists a further need for an efficient isolator that is not limited by dielectric-breakdown limits. There exists a further need for a beam source incorporating the efficient isolator.